Hypothesis for the origin of anomalous VP/VS ratios in GOM subsalt mudrocks: Implications for pore pressure prediction

Abstract

Acoustic properties of subsalt mudrocks from the Gulf of Mexico (GOM) often display unusual VP/VS ratio behavior that has been attributed to differences in the vertical and horizontal stress states below base salt. The observed VP/VS ratios are higher relative to established GOM mudrock calibrations, returning to trend with increasing depth below base salt. In the Mississippi Canyon area of the GOM, temperatures at base salt are lower by up to 40°F relative to the regional thermal gradient. The present study tested the hypothesis that the unusual mudrock acoustic properties were caused by precipitation of halite in mudrock pores due to cooling of a halite-saturated brine, caused by conductive loss of heat through overlying salt. A drop of approximately 40°F from 235°F for the normal thermal gradient, to the present-day subsalt temperature of 190°F, results in a reduction of soluble halite at saturation of up to about 30 volume percent (vol%). A Gassmann fluid substitution model in which solid halite replaces brine in mudrock pores was used to test this hypothesis. The results of the Gassmann fluid substitution model reproduce the observed acoustic log responses measured in subsalt mudrocks from prospect B, a deepwater GOM subsalt discovery. The results show that about 30 vol% solid halite could explain most of the observed deviations in VP/VS ratio, with model and observations returning to normal VP/VS behavior with 5%–10% solid halite substituted for brine at approximately 2500 ft below base salt. P-wave velocity-void ratio–vertical effective stress (VP-∊-σv) pore pressure models have traditionally fared poorly in subsalt mudrocks. Such models generally rely on VP-porosity relationships that are controlled by compaction in which the vertical effective stress, i.e., overburden stress, is the maximum historical stress. These conditions of stress and porosity are clearly not satisfied in subsalt mudrocks. Predicted velocities are too fast, inferred porosity and void ratios are too low, and subsequent estimates of pore pressure are too high. The model discussed in this paper explains the apparent disconnect between predicted and observed subsalt mudrock porosities and offers a relatively simple method for correcting them using the results of the Gassmann halite-brine fluid substitution model.